ixgbevf: allow to set MTU

ixgbevf: allow to set MTU

The support of jumbo frames in the ixgbevf Poll Mode Driver of 10GbE
82599 VF functions consists in the following enhancements:

- Implement the mtu_set function in the ixgbevf PMD, using the IXGBE_VF_SET_LPE
  request of the version 1.0 of the VF/PF mailbox API for this purpose.

- Add a detailed explanation on the VF/PF rx max frame len negotiation.

Signed-off-by: Ivan Boule <ivan.boule@6wind.com>
Signed-off-by: David Marchand <david.marchand@6wind.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>